Motor vehicle air-conditioning system and a method for operating a motor vehicle air conditioning system

ABSTRACT

An air conditioner for motor vehicles has a refrigerant circuit in which the refrigerant is brought into a wet vapor state. The refrigerant circuit is comprised among other things of at least one compressor ( 37 ) and a pressure ramming machine ( 14 ′) which serves as an expansion device. By means of the pressure ramming machine ( 14 ′), energy is recovered during the expansion process which can be used in the circuit for compressing the refrigerant. In addition, a method for operating an air conditioner for motor vehicles is described, according to which a pressure ramming machine is integrated into the refrigerant circuit and refrigerant is brought into a wet vapor state in the pressure ramming machine ( 14 ′).

PRIOR ART

The invention relates to an air conditioner for motor vehicles and amethod for operating a motor vehicle air conditioner.

Currently, air conditioners for motor vehicles almost exclusively usethe so-called cold vapor compression process with tetrafluorethane(R134a) as a refrigerant. The refrigerant circuit is comprised amongother things of an evaporator, a compressor, a liquefier, and anexpansion valve. An isenthalpic throttling of the refrigerant occurs inthe expansion valve. An isentropic, ideal expansion can only beapproached with the aid of an expansion engine; such machines have notbeen used for a long time in vehicle air conditioners. There are severalreasons for this. For one thing, the energetic improvement which couldhave been achieved by means of expansion engines was relatively low andwas not in any proportion with the higher cost in comparison to a simpleexpansion valve. For another thing, the expansion engines that werepreviously possible could only be controlled with a considerableincrease in technical expense; a speed regulation or a control of theinlet and outlet valves would be required for this. Primarily, with themachines that were previously possible, rapid destruction or a highamount of wear during operation was to be expected, with refrigerantexpanding out of the fluid.

The object of the invention is to produce an air conditioner forvehicles which, with a design cost that is only slightly higher thanconventional air conditioners, is distinguished by a significantincrease in performance. In addition, a method for operating an airconditioner should be disclosed, which results in a higher performancenumber.

ADVANTAGES OF THE INVENTION

The object of improving an air conditioner is attained by virtue of thefact that the air conditioner has a refrigerant circuit in whichrefrigerant is brought into a wet vapor state and has at least onecompressor and an expansion device which is embodied as a pressureramming machine.

The object of producing a novel method is attained according to theinvention by virtue of the fact that a pressure ramming machine isintegrated into a refrigerant circuit and refrigerant is brought atleast partially into a wet vapor state in the pressure ramming machine.The refrigerant is expanded from a high pressure level to a low pressurelevel. Preferably, refrigerant is simultaneously compressed from a lowpressure level to a high pressure level.

A pressure ramming machine is distinguished by a simple manufacture. Therotor can, for example, be comprised of extruded profiles and thehousing parts can be milled out of aluminum or produced as diecastparts. The rotor speed, which is on an order of magnitude of e.g. 10,000to 20,000 rpm results in the fact that no particularly high demands areplaced on strength. In contrast to other expansion engines, in pressureramming machines, no control or regulation is required, preferably noteven in the inflow and the outflow of the refrigerant. Since no valvesare required, the air conditioner can be operated with a high fluidcontent in the wet vapor zone. Furthermore, the pressure ramming machinepermits operation without a drive mechanism because a pressure rammingmachine can be driven exclusively by the impetus forces of the substanceexpanding inside it, in this instance a refrigerant, as experiments haveshown. With the pressure ramming machine, an expansion can and does takeplace, preferably even in the wet vapor zone, which has until now beenavoided in expansion engines due to their susceptibility in terms of thedestruction of or wear on the moving parts, in particular the blades inexpansion turbines. For this reason, even expansion engines are almostexclusively used in cold gas processes where such problems do not occur.

Advantageous embodiments of the invention constitute the subjects of thedependent claims.

Preferably, the refrigerant is carbon dioxide which in the refrigerantcircuit, e.g. when used in vehicle air conditioners, is brought at leasttemporarily from a supercritical state into a wet vapor state duringexpansion, independently of the ambient temperature. This embodiment ofthe air conditioner according to the invention is particularlysignificant. The combination of carbon dioxide and the use of a pressureramming machine results in significant advantages. With carbon dioxideas a refrigerant, greater energetic improvements and an increase incooling performance can be achieved by an expansion engine, namely incomparison to the tetrafluorethane used previously. According to apreferred embodiment, the air conditioner according to the invention canhave a transcritical or subcritical process guidance, depending on thetemperature of the heat sink, i.e. the ambient temperature duringoperation of the air conditioner or the internal temperature duringoperation of the heat pump. Since the carbon dioxide has a criticalpoint of approximately 31° C., operating conditions can occur in whichthe refrigerant circuit runs in the transcritical range, with anexpansion from the supercritical into the wet vapor state or runs in thesubcritical range in which the refrigerant can be converted from a fluidstate into a wet vapor state.

According to a preferred embodiment, the pressure ramming machine andcompressor are connected to each other so that power released during theexpansion process in the pressure ramming machine is used to compressthe refrigerant, as a result of which the drive mechanism for thecompressor can be considerably smaller in size. The cost for thepressure ramming machine is thus at least partially recouped. Theexpansion work is advantageously at least partially reused in thepressure ramming machine itself as compression work so that the drivework still required for the additional compressor still required isconsiderably lower in comparison to an operation without a pressureramming machine. In this embodiment, the compressor section of thepressure ramming machine is correspondingly integrated into therefrigerant circuit. However, it is also possible to provide severalrefrigerant circuits or branched refrigerant circuits; the compressorsection is then integrated into one of these circuits and therefrigerant can mix with the medium of the circuit into which thecompressor section is integrated.

The compressor, which is provided as an auxiliary compressor in additionto the pressure ramming machine, is of a lower performance than arefrigerant circuit without a pressure ramming machine.

The auxiliary compressor can be connected in series or in parallel withthe compressor section of the pressure ramming machine.

According to a preferred embodiment, the pressure ramming machine isdriven exclusively by the impetus forces of the mass flows ofrefrigerant flowing through it, for which, in principle, only a very lowdrive power is required.

One embodiment of the method according to the invention provides for theuse of carbon dioxide as a refrigerant, which can be brought into a wetvapor state in the pressure ramming machine depending on the ambienttemperature.

Other features and advantages of the invention ensue from the followingdescription and the following drawings which are referred to in thedescription.

DRAWINGS

FIG. 1 depicts a first embodiment of the air conditioner according tothe invention, which has a pressure ramming machine, showing the form ofits refrigerant circuit,

FIG. 2 schematically depicts a pressure ramming machine of the kind thatis used in the air conditioner according to the invention,

FIG. 3 shows a second embodiment of the air conditioner according to theinvention,

FIG. 4 shows a pressure/enthalpy graph for the air conditioner accordingto the invention according to FIG. 3 during subcritical process guidanceand without the influence of the auxiliary compressor,

FIG. 5 shows a pressure/enthalpy graph for the air conditioner accordingto the invention during transcritical process guidance, in a depictionwithout a division of the compression work W_(v) into recoveredexpansion work W_(Ex) and the work W_(vz) of the auxiliary compressor.

FIG. 6 shows a circumferential developed view of the rotor of thepressure ramming machine shown in FIG. 2, and

FIG. 7 shows a third embodiment of the air conditioner according to theinvention.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

FIG. 1 depicts an air conditioner for a motor vehicle by showing itsrefrigerant circuit. Carbon dioxide is used as a refrigerant and can bebrought to a supercritical pressure. Listed in the flow direction, therefrigerant circuit includes a compressor 10, a gas cooler or liquefier12, a pressure ramming machine 14 that constitutes an expansion device,and an evaporator 16. These elements are connected to one another vialines. In this embodiment, the pressure ramming machine 14 has anexpansion side and a compressor side, as described in detail below inconjunction with FIG. 2. The expansion side is integrated into therefrigerant circuit shown and the compressor side is integrated into asecond circuit that is not shown in detail; the second circuit does notabsolutely have to be used for the air conditioning or is usedexclusively for air conditioning the passenger compartment. Therefrigerant of both circuits can partially mix in this embodiment.

The air conditioner according to the invention works as follows: in thecompressor 10, the carbon dioxide is compressed in the process of whichthe compressor 10 absorbs an energy W_(v′). In the subsequent gas cooleror liquefier 12, the heat energy Q_(C) is extracted from the carbondioxide. When used in vehicle air conditioners, the carbon dioxide isfrequently in the supercritical state during year-round use. In thesubsequent pressure ramming machine 14, the carbon dioxide expands intothe wet vapor range in the course of which an energy W_(Ex) is recoveredby the pressure ramming machine. This energy W_(Ex) is used in thecompressor section to compress refrigerant, in the process of which thecompressor section requires an energy W_(v) for its operation, which issupplied by the energy W_(Ex). In the evaporator 16, ore energy Q_(o) issupplied to the carbon dioxide so that the carbon dioxide then assumes astate close to or at the threshold between wet vapor and vapor or is inpurely vaporous form.

The pressure ramming machine 14 is shown in FIG. 2. In one possibleembodiment, the pressure ramming machine 14 has a first housing side,referred to below as the expansion side 18, and disposed axiallyopposite from this, has a second housing side, referred to below as thecompressor side 20, as well as a rotor 22 disposed between them, whichis shown in a partially sectional view.

On the compressor side 20, the pressure ramming machine 14 has an inlet24 and an outlet 26, depicted by respective arrows and correspondingconduits. An inlet 28 and an outlet 30 are likewise provided on theexpansion side 18, likewise indicated by arrows and conduits. It shouldbe noted that the conduits for the inlet 24 and the outlet 30 have agreater cross section than the conduits for the outlet 26 and the inlet28.

Since both an expansion and a compression take place in the rammingmachine 14, the embodiment shown in FIG. 1 can be further modified inthe manner shown in FIG. 3 so that the pressure ramming machine 14simultaneously also functions as a compressor in the refrigerantcircuit. The use of the energy W_(Ex) for compression consequently alsotakes place inside the pressure ramming machine 14 itself. In order todifferentiate from the embodiment according to FIG. 1, the pressureramming machine in FIG. 3 is labeled 14′ and is alternatively likewiselabeled 14′ in FIG. 2. Downstream of the evaporator 16, a line 32 leadsindirectly into the pressure ramming machine 14′, in other words intoits compressor section between the inlet 24 and the outlet 26, and leadsto the liquefier 12 by means of a line 34 for compressed refrigerant.Parallel to the compressor section in the pressure ramming machine 14′,an auxiliary compressor 37 is provided, which consumes an energy W_(v″)during its operation.

In the pressure/enthalpy graph shown in FIG. 4, the supplied anddischarged energy quantities described above are depicted by way ofexample for a subcritical process guidance; for the sake of simplicity,the influence of the auxiliary compressor is not taken into account.From point 1 to point 2, the refrigerant is compressed through thesupply of the energy W_(v′); from point 2 to point 3 in the liquefier12, the refrigerant is liquefied both isobarically and by being suppliedwith the energy Q_(C) or is brought to the threshold between fluid andwet vapor, from point 3 to point 4, the refrigerant is released into thewet vapor range through the delivery of the energy W_(Ex), and finally,from point 4 to point 1, the refrigerant is converted completely intothe vaporous state in the evaporator through the consumption of theenergy Q_(o). The energy recovery that can be achieved with the airconditioner according to the invention in comparison to the airconditioners provided with an expansion valve can be clearly inferredfrom FIG. 4. The dashed line 5 namely depicts the isenthalpic expansionwhich the refrigerant undergoes from point 3 to point 4′ in theexpansion valve. No additional energy W_(Ex) can be recovered from theprior circuit in order to drive a compressor, for example. The airconditioner according to the invention comes significantly closer to theideal circular process which is characterized by an isentropic expansionfrom point 3 to point 4″, which is symbolically depicted by the line 6.In FIG. 4, it is also clear that with the air conditioning systemaccording to the invention, the energy consumption W_(v), in thecompressor 10 remains unchanged in comparison to the prior process,while the usable refrigerating energy Q_(o) increases by the amountW_(Ex). Precisely stated, the energy quantities shown in thepressure/enthalpy graph are quantities of specific energy.

The air conditioner according to the invention can have not only asubcritical process guidance but also a transcritical process guidanceas shown in the graph according to FIG. 5, where for the sake ofsimplicity, the influence of the auxiliary compressor is not shown.Whether a subcritical or supercritical process is required is afunction, among other things, of the ambient temperature.

The reference numerals already used in connection with FIG. 4 are usedagain in FIG. 5. In the process course shown, the point 1 is disposed onthe saturation line between the vaporous and wet vapor states. Frompoint 1 to point 2, the refrigerant is compressed through the supply ofthe energy W_(v′), where dashed lines are used to also depict anisentropic compression through a supply of energy W_(vis). At point 2,the refrigerant is in the supercritical range which it also does notleave when it flows through the gas cooler/liquefier 12 and undergoes anisobaric state change until it reaches the point 3 and gives off theenergy Q_(C). From point 3 to point 4, the refrigerant in the pressureramming machine 14′ is released into the wet vapor range while givingoff the energy W_(Ex). W_(Exis) symbolizes the energy which would haveto be produced in an isotropic expansion. From point 4 to point 1, therefrigerant passes through the evaporator in which it absorbs energy.

Based on FIGS. 2 and 6, the function of the pressure ramming machine 14′can now be explained in brief. For the sake of better comprehension, thereference numerals for the inlets and outlets 24 to 30 used inconnection with FIG. 2 have also been used in FIGS. 3 and 6.

The states inside a so-called cell, i.e. the space between neighboringblades of the rotor, during one rotation are described below; the statesof the refrigerant in the cell change during the rotation. In FIG. 6, acell is symbolically provided with the reference numeral 36. The cell 36is moved from bottom to top in the developed view according to FIG. 6.For the sake of clarity, it is first assumed that the ambient pressureand ambient temperature prevail in a refrigerant that is at rest in thecell. When the cell 36 has reached the opening edge 38 of the inlet 28,a pressure wave is introduced into the cell 36 because the pressure ofthe refrigerant in the inlet 28 is greater than that in the cell 36. Thepressure wave moves into the cell 36 at the speed c_(d). The total speedof the pressure wave is symbolized by a line 40, which is produced bysuperposing the speed c_(d) and the circumference speed u. Therefrigerant disposed in the cell 36 upon reaching the opening edge 38 issubjected to an abrupt pressure and speed increase. A temperatureincrease must also be recorded. The refrigerant that is to be expandedflows from the inlet 28 into the cell 26 and the then-compressedrefrigerant then flows out of the cell 36 via the outlet 26. When thepressure wave reaches the compressor side 20, the entire content of thecell has been accelerated to the speed C₃. The superposition of thespeed C₃ and the circumference speed u yields the imaginary dividingline 42 depicted with dashed lines in FIG. 6, which constitutes theboundary between the refrigerant flowing in via the inlet 28 and thecompressed, escaping refrigerant that was originally disposed in thecell 36. The closing edge 44 for the outlet 26 must be disposed so thatit coincides with the arrival the dividing line 42 so that noundesirable reflection of the pressure waves occurs on the compressorside 20 and a maximal refrigerant quantity is expanded and compressed.

After the cell 36 closes, it conveys the refrigerant, which has flowedin, further along. Because of the impetus of the refrigerant and thepropagating pressure wave, the pressure in the cell 36 decreases on theexpansion side 18. In the cell 36, at 18 and 20 there is an axial speedc of approximately 0. Upon reaching the opening edge 46 of the outlet30, in the ideal case the pressure p₄ prevails here. Due to thereflection of the pressure wave against the side 20 and the expansion ofthe refrigerant, the compressed refrigerant flows out. When the openingedge 48 of the inlet 24 is reached, refrigerant, which is to becompressed, arrives in a replenishing flow because it is sucked into thecell 36 as a result of the gas impetus. The imaginary dividing line 50separates the outflowing refrigerant from the incoming. The correctlytimed delay of the flow and the prevention of a reflection can beprevented by the correct disposition of the closing edges 52 and 54;here, too, the disposition of the flow edges 52 is once more matched tothe speed of the outflowing gas c₄ and the circumference speed u.

The pressure compensation otherwise takes place inside a cell at thespeed of sound, which is why the pressure ramming machine 14′ has a highperformance at rotor speeds of e.g. only 10,000 to 20,000 rpm, which areadvantageously low for a flow machine of this performance class.

Although an external drive mechanism can naturally be provided for therotor, embodiments can also be achieved in which merely the impetusforces of the refrigerant flowing into the pressure ramming machine 14′are sufficient to drive the rotor 22.

Instead of the parallel connection of the auxiliary compressor 37, itcan also be connected in series with the compressor section of thepressure ramming machine 14′, as shown in FIG. 7. The entire refrigerantflow is brought to a first pressure level by the pressure rammingmachine 14′ and then in the auxiliary compressor 37, it is brought tothe pressure that is required in the liquefier or gas cooler 12. Theembodiment according to FIG. 7 has the advantage over the one shown inFIG. 3 that fewer lines are provided and fewer internal leakage lossesare expected as a result of the lower pressure difference. In addition,the final compression temperature in the machine is lower, as a resultof which fewer thermodynamic losses are produced. It is alsoadvantageous that the machine can be freely placed in the vicinity ofthe compressor without greater deviations from conventional systemconnections.

As can be seen from the embodiments shown, there is the advantage thatno valve is provided, which makes the air conditioner very favorable andis a prerequisite for a low susceptibility to malfunction.

The specialist can easily deduce heat pump connections from the systemconnections shown, where corresponding reversing valves must be providedfor the alternative heating, cooling, or dehumidifying operation.

What is claimed is:
 1. An air conditioner for motor vehicles, with arefrigerant circuit In which refrigerant flows, said circuit comprisingat least one compressor (10; 37) and a pressure ramming machine (14;14′), which constitutes an expansion device, wherein said refrigerantwithin said circuit is brought into a wet vapor state within thepressure ramming machine (14; 14′).
 2. The air conditioner according toclaim 1, characterized in that the refrigerant is carbon dioxide and theair conditioner is designed so that the carbon dioxide can be broughtfrom a supercritical state into a wet vapor state in the refrigerantcircuit.
 3. The air conditioner according to claim 2, characterized inthat the refrigerant circuit has a transcritical or subcritical processguidance depending on the ambient temperature.
 4. The air conditioneraccording to claim 1, characterized in that the pressure ramming machine(14) and the compressor (10) are connected to each other so that energyreleased during the expansion process in the pressure ramming machine(14; 14′) is used for the compression (10).
 5. The air conditioneraccording to claim 1, characterized in that the pressure ramming machine(14′) has a compressor section and this compressor section is integratedinto the refrigerant circuit so that the pressure ramming machine (14′)simultaneously also constitutes the compressor.
 6. The air conditioneraccording to claim 5, characterized by means of an auxiliary compressor(37) which is connected in series with the compressor section of thepressure ramming machine (14′).
 7. The air conditioner according toclaim 6, characterized in that the auxiliary compressor (37) is disposeddownstream of the compressor section.
 8. The air conditioner accordingto claim 5, characterized by means of an auxiliary compressor (37) whichis connected in parallel with the compressor section of the pressureramming machine (14′).
 9. The air conditioner according to claim 1,characterized in that the pressure ramming machine (14; 14′) is drivenexclusively by impetus forces of the refrigerant expanding inside it.10. The air conditioner according to claim 1, characterized in that therefrigerant circuit is embodied so that the air conditioner canalternatively be operated as a heat pump.
 11. A method for operating anair conditioner for motor vehicles, characterized by the followingsteps: a pressure ramming machine (14; 14′) is integrated into therefrigerant circuit and refrigerant is brought at least partially into awet vapor state in the pressure ramming machine (14; 14′).
 12. Themethod according to claim 11, characterized in that carbon dioxide isused as the refrigerant and can be brought from a supercritical stateinto a wet vapor state in the pressure ramming machine (14; 14′),depending on the ambient temperature.
 13. The method according to claim12, characterized in that a transcritical or subcritical processguidance is executed depending on the ambient temperature.